Any given (date+)time value is a floating point number with time in days, e.g. 0.5 == 12h. To convert the value displayed as time simply multiply it by 86400 to get the number of seconds, and to convert an amount of seconds to display it as time divide by 86400 (and best use the duration format [HH]:MM:SS
so the hours aren’t modulo 24 and proper rounding is applied).
However, it’s not quite clear to me what your (30 / 1sec)
actually should achieve. 1sec is what? And what unit are the 30 supposed to be? (You could clarify by editing your question). Anyway, to obtain the given result 45.095 the formula would be
=A2*86400*30/1000/A1
However, that is not in seconds (like you stated with X=45.095 sec.) but in “thirty-millis” or “three-hundreths”, so to convert back to reach a result of 12:31:35 the formula (assuming the previous formula to be in A3 here) is
=A3*1000/86400
Looks a bit odd to me, but if that is what you want…